Achieving good Image Rejection (IR) performance in heterodyne receivers is one of the most important challenges in high performance radio design and the choice of radio architecture used in many applications is very often dictated by the image rejection requirements of the overall system.
For example, in a double superheterodyne architecture, careful consideration must be given to proper frequency planning to achieve good IR performance. In this architecture, the use of a high first IF frequency relaxes the constraints on the RF band select filter at the low noise amplifier (LNA) input and improves image rejection performance. However, this comes at a cost of more expensive and power hungry filters for the first IF stage. On the other hand, a low first IF frequency relaxes the bandwidth, power and cost constraints on the first IF filter but now the external band select filter at the LNA input must have a much higher Q factor to maintain good image rejection performance. Similar consideration must also be given to the selection of the second IF frequency in a double superheterodyne design which also has an image component. In general, superheterodyne receivers can be designed to have excellent selectivity and can exhibit very good image rejection performance, but this comes at a cost of power and complexity and they are not widely used in integrated low power radio designs.
A zero-IF receiver has the primary advantage that it does not have an image component. However, the zero-IF architecture is prone to low frequency impairments such as 1/f noise and DC offset problems and is not suitable for narrowband wireless communication applications such as the Flex/ReFlex pager standards and PMR radio standards such as APC025 and TETRA, where occupied spectral bandwidths of 6.25 kHz, 12 kHz and 25 kHz are required.
Narrowband wireless telemetry and wireless sensor applications are other examples of communication networks where zero-IF receivers are not widely used. For example, the regulatory bodies; FCC (USA), ETSI (Europe) and ARIB (Japan) permit narrowband wireless telemetry in selected RF bands. In the USA, compliance to FCC part 90 requires channel bandwidths of 6.25-25 kHz channels. In Europe and in Japan, specifications for ETSI EN300-220 and ARIB STD-T67 respectively, require channel bandwidths of 12.5-25 kHz.
A low-IF receiver architecture overcomes the low frequency and 1/f noise problems of the Zero-IF receiver by moving the received spectrum away from DC and this receiver architecture is well suited to the narrowband wireless telemetry applications described above. In a low-IF receiver architecture, image rejection is typically accomplished by the use of Hartley or Weaver image rejection techniques or by the use of complex analog bandpass filters. However, these architectures suffer from poor to moderate image rejection due to quadrature gain and phase mismatch errors in the local oscillator (LO) and signal paths. Fundamentally, these methods rely on complex signal cancellation techniques to remove the image component. However, due to manufacturing process tolerances, it is difficult to ensure quadrature gain and phase errors of better than 1-2% and 1-3 degrees respectively, which results in a typical image rejection performance of 25-30 dB.
Thus, there is a need to enhance the IR performance of low-IF receiver architectures, and still preserve the benefits of low power, low-complexity and excellent narrowband performance that the low-IF receiver architectures offers.